This invention concerns the purification of exhaust gases, especially the purification of such gases from diesel and other xe2x80x9clean-burnxe2x80x9d engines.
Diesel engines are in widespread use in all types of vehicles, stationary power sources and naval and commercial shipping. They are very fuel-efficient, but because of their combustion characteristics generate particulate matter (soot, often called xe2x80x9cPMxe2x80x9d) on which a variety of organic substances may be absorbed, including unburnt hydrocarbons (HC) and sulphuric acid produced by oxidation of sulphur dioxide derived from sulphur species present in the fuel or in lubricants. Other engines, such as gasoline direct injection (xe2x80x9cGDIxe2x80x9d), can also produce significant quantities of PM, and we consider that the need for removing such PM will soon be expressed in legislation. Nonetheless, the invention may be applied to combustion processes generally, as well as potentially to chemical process stacks/exhausts, and to combustion engines operating at xcex=1 or greater, or lean-bum engines operating at stoichiometric or rich in order to regenerate some exhaust gas aftertreatment device. For simplicity, however, we concentrate on diesel engines hereinafter.
In order to meet various regulations concerning the level of pollutants, it has become commonplace to fit vehicles with an oxidation or three-way catalyst, which only achieves partial removal of PM. The removal of particulates is generally achieved by using some form of filter or trap, which may be cleaned or regenerated intermittently. It has been suggested to include a catalyst in the fuel to the engine, and as well as platinum group metals (xe2x80x9cPGMsxe2x80x9d), iron, copper or cerium compounds have been suggested. A particulate trap may be catalysed to lower the soot combustion temperature, and some form of external heating, for example electric heating of the trap or of air fed thereto, may be used to initiate soot combustion.
A particularly successful soot trap is marketed by Johnson Matthey PLC as the xe2x80x9cCRTxe2x80x9d (xe2x80x9cContinuously Regenerating Technologyxe2x80x9d) and is described in U.S. Pat. No. 4,902,487. This system uses a conversion of NO in the exhaust gas to NO2, which was discovered to be much more effective at typical low diesel exhaust gas temperatures in the combustion of soot than air or any other exhaust gas component. Thus, NO2 is typically effective to combust PM at about 250xc2x0 C., whereas oxygen is effective at about 650xc2x0 C.
It has been suggested to use a plasma generator for exhaust gas purification (see for example GB 2,274,412 and 2 270 013, UK Atomic Energy Authority). Although it was probably not previously recognised in connection with exhaust gas treatment, such a system produces considerable quantities of NO2. Systems such as previously described do not include any filter or trap in combination with a plasma generator, but we believe that this may be a particularly effective system for treating diesel and similar lean-bum exhaust gases.
Accordingly, the present invention provides a system for the treatment of such exhaust gases, comprising a plasma generator effective to convert at least a portion of NO and/or nitrogen in the exhaust gases to NO2 and/or to generate ozone and a filter effective to trap a desired proportion of soot from the exhaust gases, whereby said trapped soot is combusted by reaction with NO2 and/or ozone, at a markedly lower temperature than required by O2.
We believe, although we do not wish to be bound by any theory, that in the present invention NO2 may be generated not only by oxidation of NO in the exhaust gases, but also by oxidation of nitrogen to yield NO, which is itself converted to NO2. In the latter case, there is no reliance upon the quantities of NOx leaving the engine. It is also believed that the present invention is especially valuable in that it is not adversely affected by the presence of sulphur in the fuel or in lubricants, which can poison conventional catalysts.
The invention further provides a method of reducing emissions from exhaust gases from diesel and like engines, comprising trapping soot on a filter and continuously or intermittently combusting the soot by reaction with NO2 and/or ozone produced by a plasma generator, preferably using a plasma treatment of at least a portion of the exhaust gases.
The plasma generator may be any suitable type producing a non-thermal plasma, and may be enhanced by electromagnetic radiation. Suitable plasma generators include high voltage (e.g. 20 kV or more) alternating current, preferably pulsed, generators, suitably using two dielectric plates positioned in the gas flow, and piezoelectric devices such as piezoceramic transformers. It may be positioned to treat all or a portion of the exhaust gases upstream of the filter, or may be fitted downstream of the filter to treat all or a portion of the filtered exhaust gas, with recirculation of plasma-treated gases to the filter. In one embodiment of the present invention, a pre-determined proportion of the exhaust gases is treated by the plasma to cause substantially all of the NO present to be converted into NO2, and the resulting gases blended with untreated exhaust gases, thus resulting in a blend of NO and NO2, which according to some studies, may be more effective for the purposes of the present invention than a gas containing substantially only NO2 in admixture with other exhaust gas components.
The filter used may be a woven or knitted wire filter, a gas-permeable metal or ceramic foamed mass or a wall flow filter of generally known type (honeycomb monolith). For certain vehicles, especially light cars or vans, it may be necessary or desirable, to use a filter design which collects only 80% or so by weight of the total soot particulates and preferably incorporates a by-pass and/or pressure relief valve. The filter may be partially or completely catalysed if desired. A catalysed trap may improve the aggregate removal of pollutants.
A modification of the present invention incorporates a means for removing NOx downstream of the filter and plasma generator. Such means may be a NOx trap, which technology is available to the skilled person, and generally includes one or more alkali earth metal compounds, especially calcium oxide or barium oxide, or alkali metal, carried on a metal or ceramic honeycomb-type support. The NOx trap is desirably used in combination with a lean-NOx catalyst. Another means for removing NOx is Selective Catalytic Reduction (xe2x80x9cSCRxe2x80x9d), which is well established for stationary power sources and is receiving increasing attention for vehicular applications. Such a modified system can be effective to meet all current and known future emission control regulations for diesel and like engines.
The plasma generator may be controlled and actuated by an engine management unit, or other microprocessor control unit, to operate intermittently according to certain engine operating conditions (speed, load etc) which have been pre-determined to generate more soot. The identification of such operating conditions can be readily accomplished by one skilled in the art, and such conditions include times when the emission of particulate matter would be undesirably high in the absence of any treatment therefor. Alternatively, the plasma generator may operate during all operational conditions of the engine, which system has the benefit of simplicity, but this may be undesirable if the engine is in an operating condition in which significant quantities of NOx are generated, or during regeneration of a NOx trap.
The present invention, at least in its most preferred embodiments, in addition to being particularly effective at controlling emissions, permits the engine designers to design and tune the engine for power and/or fuel efficiency, rather than being forced to make compromises in engine design to minimise the generation of NOx and particulates. This can be a significant advantage for commercial vehicles, but allows flexibility in design for all engines and types of vehicles.
A further variant of the present invention is to feed a reductant, which term includes hydrocarbon fuel, e.g. diesel fuel, ammonia, ammonia precursors, hydrogen etc. into the exhaust gases either upstream or downstream of the plasma generator.
The present invention is illustrated with reference to the accompanying drawing, which is a schematic diagram of a system according to the invention.
A diesel engine is shown at 1, and has an exhaust system, 2. Conventional silencer boxes and ancilliary equipment are not shown. A wall flow filter, 3, retained within a metal box, 4, is mounted in the exhaust system. Mounted close upstream to the filter, is a plasma generator, 5, which is operated according to signals from the engine management unit, 6.
Testing of the described system is continuing, but early indications are that substantially all soot particles trapped on the filter are removed continuously, although there are variations in soot build up and removal rates. NO2 and ozone have been detected in the exhaust gases after the plasma generator, with substantially lower levels after the filter.
The following Examples illustrate features of the present invention.